The invention relates to a drill head for drilling a bore in the ground, as usable, for example, in the so-called HDD (horizontal direction drilling) methods. The drill head has a first unit coupleable with an inner drill pipe of the double drill pipe assembly and a second unit coupleable with an outer drill pipe of the double drill pipe assembly. By means of the inner drill pipe the first unit is rotatable about an axis of rotation relative to the second unit which is drivable by means of the outer drill pipe. Further, the invention relates to a device for drilling a bore in the ground by means of such a drill head.
By means of drill heads so-called pilot bores between a starting point and a target point can be made, which can then be enlarged by means of a second drill head and/or a back reamer. Dependent on the nature of the ground through which the bore is to be drilled, different drill heads are used, such as so-called rock reamers which, for example, comprise a roller bit arrangement or so-called impact moles or down-the-hole hammers by means of which a hammer drilling operation is performed. Both the pilot bores as well as the reaming of a bore by means of a reaming tool can take place in a liquid-supported manner, in particular supported by a bentonite rinsing liquid. By means of the rinsing liquid the excavated earth is removed from the bore channel. Pilot bores and reaming bores can be made, for example, by means of a horizontal drilling device sold by the applicant under the name “TERRA-JET”. By means of this drilling device horizontal direction-controlled drillings are possible. For this, the drill head, by means of which the pilot bore is made, usually has a battery-powered trackable transmitter so that its position can exactly be determined from the earth's surface by means of a portable locating device. For this, however, the locating device has to be directly vertically above the sonde. Further, the drilling depth must not be too deep so that the signal emitted by the sonde reaches the earth's surface. Based on the determined position of the sonde, the drill head can be controlled such that a desired path of the drilling to be made is achieved. The first unit driven by the inner drill pipe can have means for the excavation of the earth. For example, rock drill heads are known which are provided with three roller bits that are each equipped with hard metal pins. Such rock drill heads are rotated at 30 to 300 rpm. In doing so, the roller bits roll upon the earth, in particular upon the rocks. The surface pressure between the hard metal pins of the roller bit and the rock causes that rock pieces, so-called cuttings, are broken away and then removed backward out of the bore channel by means of the rinsing liquid. At the same time, the bentonite rinsing liquid stabilizes the bore channel.
When using a pneumatic down-the-hole hammer as a drill head, compressed air flows through the inner bore of the inner drill pipe into the down-the-hole hammer and drives the hammer ram of the down-the-hole hammer with up to 2500 impacts per minute. The hammer ram hammers directly onto the rock drill head, the hard metal pins of which excavate the rock by means of the impacts. The exhaust air of the down-the-hole hammer blows the excavated earth or rock particles (cuttings) backward through the bore channel.
For performing a controlled drilling by means of these drill heads, such as a controlled horizontal drilling, the center axis of the drill head is curved or bent. The bend or the curvature is usually in the range between 1° and 3°. When the drill head is driven by means of the inner drill pipe and when the casing of the drill head is simultaneously pressed into the ground without rotation, the drill head moves along a curved bore along its bend. For drilling in a straight line, the casing of the drill head has to be slowly rotated continuously. Usually, the casing of the drill head is rotated at 10 to 30 rpm to obtain a straight bore. In the case of down-the-hole hammers, the controlled bore can, alternatively to the provision of the curvature or a bend in the center axis of the down-the-hole hammer, also be made by means of a design of the excavation area of the drill head, preferably equipped with hard metal pins, which is asymmetrical with respect to the center axis. Here, the excavation area is provided at the front end of the drill head and is in direct contact with the earth to be excavated. By way of the asymmetrical design of the excavation area, the drill head would produce a non-linear bore without a rotation of the down-the-hole hammer by means of the outer drill pipe. To produce a linear bore by means of the down-the-hole hammer, this hammer has to be slowly rotated continuously by means of the outer drill pipe, preferably at 30 to 50 rpm.
For locating the rock drill head in the ground, the electronic sonde already mentioned is inserted into a sonde chamber in the outer casing of the drill head which is rotatable by means of the outer drill pipe. The sonde comprises a battery or an accumulator for its energy supply. Preferably, the sonde is protected against shocks and impacts by damping elements. The sonde chamber formed by an opening is covered with a cover plate in which a relatively large slot is formed through which the sonde signal can reach the outside. Alternatively, the cover plate may also be made of a material, such as plastic, which does not shield the sonde signal. Further, a slot provided in a metal cover plate can be filled or covered by plastic.
For locating the sonde, usually an employee walks vertically above the drill head with a locating device and locates the drill head. If, however, the locating depth becomes too deep or if strong disturbing signals are present in the ground, a locating from above is no longer possible. Likewise, the battery life may be too short for the duration of the drilling so that it is not possible to locate a battery-powered sonde from above. When making a bore under water or a busy road, the drill head likewise cannot be located in the described way.
From the document DE 100 05 475 A1, a drilling device having an inner drill pipe and an outer drill pipe is known. For both drill pipes, one common rotating device is provided which drives the drill pipes synchronously. The drill pipes are driven via separate driving gears, each of which being a component part of a transmission. One of the transmissions can be displaced in feed direction in order to displace the inner drill pipe relative to the outer drill pipe.
From the document DE 11 2010 003 039 T5, a vertical drilling device having a borehole tool is known, which borehole tool comprises a slip ring arrangement that is inserted radially between a shaft and a casing. The slip ring arrangement is configured to provide various electrical communication channels between the shaft and the casing.